1. Field of Invention
The present invention relates generally to actuators. More particularly, the present invention relates to a radially symmetric voice coil motor which utilizes a magnet array which includes wedge-shaped cylindrical magnets.
2. Description of the Related Art
For precision instruments such as photolithography machines which are used in semiconductor processing, factors which affect the performance, e.g., accuracy, of the precision instrument generally must be dealt with and, insofar as possible, eliminated. When the performance of a precision instrument such a wafer scanning stage or a reticle scanning stage is adversely affected, products formed using the precision instrument may be improperly formed and, hence, function improperly.
Voice coil motors (VCMs) are often used in lithography machines to enable accurate movement of a wafer scanning stage along one axis. A VCM utilizes a magnetic coil that is placed in a magnetic field. When current is applied to the coil, a force is generated between the coil and the magnetic field predominantly in a single degree of freedom. Small VCMs are capable of generating force in one direction with a relatively high efficiency.
When a VCM is to be used in a system that is sensitive to heat such as a lithography system, excessive heat may cause the accuracy with which a lithography process may be performed to be compromised. In addition, excessive heat may cause any insulating material in coils of a VCM, as well as the copper from which coils are often formed, to melt. The amount of force that may be generated by a VCM is also limited by the amount of current which may be supplied by an amplifier, or by electronics which are used to control the VCM.
Therefore, what is needed is a method and an apparatus for enabling the force generated by a VCM to be increased without significantly increasing the current usage of the VCM or the heat output of the VCM.